Hi Kate, Sorry to hear about your Aunt. You will probably be better posting this question in the law section - there are some knowledgable bods in there.
Certainly under Scottish Law, the grandchildren would be her dependants, the sister in law would not be. Your next of kin is who you say it is and does not necessarily have anything to do with who is entitled to benefit from your aunts estate.
Equity release contracts can operate in a number of ways, you would need to have a look at the policy documents, or contact the product provider if you have Power of Attorney for your Aunt. If you don't hold this at the meoment, it would be worthwhile arranging this in order to ascertain more details of your aunts affairs - if your aunt is still able to grant you this. Having said that, generally they would expect to take the property on your Aunts death and her decendants would have no rights to live there.